Quantitative separation of benthic diatoms from sediments using density gradient centrifugation in the colloidal silica Ludox-TM

A quantitative density separation method for benthic diatoms is described in detail. Both epipelic and epipsammic diatoms have been separated from inorganic sediment components including empty diatom frustules. The method is based on differences in specific weight, using medium speed centrifugation on a step gradient of the colloidal silica Ludox-TM. Results obtained with this new method have been compared with those obtained by the lens tissue method. For the density method, applied to samples preserved with a mixture of acrolein, glutaraldehyde and tannic acid, a mean recovery of 82.3% chlorophyll a was found. In the lens tissue harvest-experiments, mean chlorophyll a harvests ranged from 29.1 to 53.2%. Moreover, the standard deviation was several times larger for the lens tissue harvest experiments than for the acrolein series processed by the density method. With the density method, sand fractions were removed entirely, while the clay fractions were removed almost completely. The remaining organic carbon (detritus), with a mean value of 42.3% of the original sample, does not interfere with diatom identification and counting, since all organic matter can be fully oxidized. The density method was found to be applicable to widely different kinds of estuarine sediment and varying quantities of benthic diatom biomass present in the samples. The method can also be used for some other algal groups such as Euglenophyceae and Cyanophyceae.Publication No. 19 of the project Biological Research in the Ems-Dollard Estuary.Communicated by O. Kinne, Hamburg